A magnetic hard disk driving unit of electronic computers and the like instruments writes and reads information through a magnetic head while a hard magnetic disk encased in a sealed container is rotated. The distance between the magnetic head and the magnetic disk is very small, and there is the possibility that the magnetic head and magnetic disk are broken to cause loss of valuable data when dust invade into the sealed container. In order to prevent such an accident, it is generally conducted that a gasket prepared by forming a macromolecular elastomer into a ring shape is placed between the main part of the container and a dust cover. As such a macromolecular elastomer, a cellular polyurethane or a cellular chloroprene rubber has heretofore been used. A gasket having a ring shape is punched out from a sheet of the above macromolecular elastomer, and the obtained gasket is attached to the container to be sealed by a mechanical method, such as fixing with screws, or by adhesion. However, these operations cause problems that the operations take considerable labor and moreover that silicone oils contained in the macromolecular elastomer as the foam stabilizer and the adhesive are vaporized and contaminate the inside of the instrument to cause error in the function of the instrument.
Using a product prepared by molding chloroprene rubber or butyl rubber together with a metal substrate as the gasket of a magnetic hard disk has been proposed. However, it is necessary for producing such a gasket that the gasket be formed by using a mold and the formed burr be removed. The operation of removing the burr causes problems that the operation takes labor and that dust is formed from the gasket when removal of the burr is insufficient, possibly causing a broken magnetic hard disk. It has also been proposed that a gasket for a magnetic hard disk is formed by coating specified parts of a dust cover with a liquid silicone rubber material and then curing the silicone rubber material (Japanese Patent Application Laid-Open No. Heisei 4(1992)-106776). However, this process causes the possibility that uncured low molecular weight siloxanes are vaporized from the silicone rubber material and condensed on the surfaces of the magnetic head and the magnetic disk.
In order to improve the above drawbacks, a gasket made of a flexible microcell polyurethane which is obtained from a polyol, an isocyanate, a foaming agent, and a silicone foam stabilizer of the reactive type and has an average cell diameter of 150 .mu.m or less has been proposed in Japanese Patent Application Laid-Open No. Heisei 5(1993)-234351. However, the problem of contamination with the residual foaming agent and foam stabilizer still remains unsolved.
For the reasons described above, development of a gasket which is easily formed and attached to a substrate, reduces loss in the used material, and does not cause contamination of a magnetic hard disk drive with dust and volatile matters is desired. Development of a gasket which can achieve complete sealing of printed circuits from dust and volatile matters in portable telephones and electronic note books is also desired by the same reasons.
Accordingly, the present invention has an object of providing a composition for a gasket which can be formed to an accurate shape, enables easy operations of forming and attaching the gasket without requiring much labor and reduction in loss of the used material, provides a gasket having a large tearing strength and a small compression set and excellent in the flexibility and the sealing property, and can prevent contamination of the inside of containers of high precision instruments with volatile matters generated from the gasket, and another object of providing a process for producing the gasket by using the above composition.